1. Field of the Invention
This invention relates to pneumatic tires and more particularly to an improvement in a pneumatic tire adapted for use in an automobile which runs at a high speed under a heavy load such, in general, as trucks, buses or trailers, all inclusive of middle type and small type, particularly a noiseless lug tire having a lug type tread pattern.
2. Description of the Prior Art
The above mentioned kind of pneumatic tire is required to have not only an excellent durability represented by resistance to wear, separation resistant property, burst resistant property or the like and an excellent dynamic ability represented by antiskid property, steering stability, rolling resistance or the like but also an excellent low noise property for the purpose of improving environmental conditions.
In order to maintain or improve a safe property of the tire, it is a matter of course that the low noise property must be improved without degrading the steering stability, antiskid property and durability.
In general, however, the low noise property is contradictory to both the durability and the dynamic properties. In addition, the durability is contradictory to the dynamic property.
As a result, it is an important problem in the techniques associated with the tire to provide a tire which can simultaneously eliminate a contradiction among these three properties and which has not only an excellent low noise property but also an excellent dynamic property and durability.
It is a matter of course that a rib type tread pattern is fundamentally useful when a low noise property is required while satisfying the desired dynamic property. But, when the rib tire is used under heavy load or a non-paved road or off-road where rocks and stones are scattered or used under any other severe conditions, rib tear, groove crack or groove base cut or burst due to stones biting into the groove and particularly a separation failure of the internal reinforcement due to overheat near the shoulder which are inherent to the rib type tread pattern are frequently induced. This forcedly degrades the durability of the tire to an inadmissible extent.
As a result, when the tire is used for the above mentioned purposes, it is indispensable to use a tire having a lug type tread pattern (hereinafter will be called as a lug tire). In the tire having the rib type tread pattern (hereinafter will be called as a rib tire), the tread continuously makes contact with ground when the tire rotates under load. On the contrary, in the lug tire, the lugs intermittently make contact with ground in succession so that noise produced when the tire runs under load becomes inevitably higher than that of the rib tire. The noise due to tires in road traffic is mainly caused by the lug tire.
Many attempts have been made to reduce the noise due to the lug tire, but hitherto none has led to fully satisfactory results due to degradation of durability and dynamic ability of the tire.
As one of such attempts, it has been proposed to make a crown radius of the tread of a tire having a size of 10,00-20, that is, a radius of curvature of an outer contour curve of the crown appeared on the radial cross sectional plane of the tire small to the order at most 250 mm. In this case, a decrease of the amount of tread rubber exerting adverse influence upon the resistance to wear of the tire functions together with a local increase of the ground contact pressure distribution at the crown center region to significantly shorten the wear life of the tire. In addition, a decrease of the ground contact area and improper ground contact configuration and improper ground contact pressure distribution result in deterioration of both the steering stability and the antiskid property of the tire, thereby exhibiting practically inadmissible drawbacks.
A mechanism of generating noise when the tire runs can be summarized as follows.
(1) When the tire makes contact with ground, the tread is deformed to compress air occluded in grooves and these cause the compressed air to expand, thereby producing a rare and dense wave, which will hereinafter be called as pumping noise.
(2) Indentations due to the presence of transverse grooves arranged on the tread cause the tread rubber to change in thickness and configuration at various circumferential portions of the tread, so that the tread and case oscillates due to synchronous shocks produced between the ground surface and the tire when it rotates, which will hereinafter be called impacting noise.
(3) Before and after the tire makes contact with ground, the tread is deformed to produce a partial slip.
The noise generated by the lug tire is mainly caused by (1) the pumping noise and (2) the impacting noise dependent on the configuration per se of the transverse groove and running speed of the tire. It has been found out that the influence of (1) the pumping noise and of (2) the impacting noise upon the noise of the lug tire are substantially the same with each other at a speed of at most 80 km/h. As a result, even though the cause of either one of (1) the pumping noise and (2) the impacting noise is eliminated, the real noise preventive effect is not sufficient enough and eventually causes the other noise to increase and hence the noise preventive effect as expected could not be obtained. As seen from the above, one of both the pumping noise (1) and the impacting noise (2) must be improved without deteriorating the other noise and without degrading the durability, steering stability and antiskid property of the tire.
Experimental tests have demonstrated the result that, in the above described kind of tire, if various dimensions for determining the plane configuration and cross-sectional plane (the cross-sectional plane in the circumferential or meridian direction of the tire, that is, the cross-sectional plane on a normal line drawn perpendicular to the outer contour curve of the tread) configuration of the transverse groove are made optimum, it is possible to overcome the conditions with respect to the low noise property, durability, steering stability and antiskid property which contradict each other.